The present invention relates to a method and an apparatus for screening or classifying fiber suspensions, i.e. for separating undesirable constituents, such as undesirable fiber fractions, fiber bundles, or impurities (e.g. debris and shives) from the fiber suspensions before further processing. The fractionation or screening process thereby separates the fiber suspension into an accept portion and a reject portion. The invention is particularly applicable to cellulose fiber suspensions (wood pulp) of low or medium consistencies (e.g. about 0.1-5%).
Screening of the fiber suspension is generally performed on fiat screen plates or screen cylinders, disposed in a screen housing. The screen plates or screen cylinders are constructed by a screening medium, having openings (i.e. holes, which are round, or slots, which are elongated) therethrough for separating the accept and reject portions of the fiber suspension.
A typical pressure screen in the pulp processing industry comprises a screen cylinder disposed in a screen housing. A rotor, constructed for operation with hydrofoils or other pulse creating devices on a solid rotating element, is disposed coaxially with the screen cylinder within the screen housing, the rotor and the screen cylinder forming a narrow screening zone therebetween. An inlet for fiber suspension is connected to the inlet end of the feed side of the screen cylinder and a reject outlet to the reject end thereof. An accept outlet is connected to the outlet side of the screen cylinder. The inlet for fiber suspension may be connected to the inner or the outer side of the screen cylinder, the inlet side of the screen cylinder thereby being formed correspondingly on the inner or outer side of the cylinder.
The screen operates completely filled with suspension (liquid and fibers), and the fiber suspension to be screened is conveyed to the inlet end of the screen cylinder into the screening zone between the rotor and the screen cylinder. The fiber suspension is thereby transported tangentially to the feed side of the screen cylinder, the rotor increasing the circumferential velocity of the fiber suspension while the suspension passes in a screw shaped flow path along the screen cylinder toward the reject end thereof.
Accept fraction is removed from the screening zone through the openings of the screen cylinder, and reject fraction is pushed forward along the feed side of the screen cylinder toward the reject outlet. The surface of the screen cylinder, i.e. the screening medium, is kept clean and unclogged by a pulsing effect created by the hydrofoils or other similar elements on the rotor, the hydrofoils moving close to the surface of the screen cylinder. Alternatively it is possible to create a pulsing effect by having a static rotor and a rotating cylinder.
It has for some time been recognized that screening mediums have a tendency to perform rather poorly in the middle of the screening zone, and especially at the outlet or reject end of the screening zone. As the fiber suspension proceeds in a spiral flow path toward the reject end of the screening zone, the motion of the rotor induces an accelerating, circumferential speed to the fiber suspension. According to the invention it has been recognized that as the circumferential speed of the fiber suspension increases along the screen cylinder, the screening capacity-affecting static pressure decreases and is offset by an increasing dynamic pressure. It therefore seems that the flow velocity of accept fiber suspension through the openings in the screen cylinder is highest at the feed end of the cylinder and lowest at the reject end thereof.
The circumferential speed of a fiber suspension, introduced at the inlet end at e.g. a circumferential speed of about 1 m/s, is considerably increased towards the end of the screening zone, whereas the difference in speed between the fiber suspension and the rotor is decreased. A rotor having a circumferential speed of about 20-25 m/s may--toward the outlet end of the screening zone--have induced almost the same speed to the fiber suspension. Due to this decrease in relative speed between rotor and fiber suspension, fiber mat destroying pulses, which are created by the rotor, decrease toward the reject end of the screen cylinder. When pushed to its limits, a conventional screen starts to thicken (dewater) the fiber suspension at the reject end. The major reason for this is believed to be the decrease in pulses or turbulence created by the rotor as the rotational speed between the rotor and the fiber suspension decreases.
As a mat of fibers is formed on the inlet (feed) side of the screening medium, the flow of accept fiber suspension through the openings (holes or slots) is further decreased. This leads in the end to complete clogging of holes adjacent the reject end of the screen cylinder. This reject end-clogging phenomena thus effectively limits the hole or slot size which would be desirable in order to optimize cleaning efficiency.
Until the invention it has not seemed possible to decrease the dimensions of the screening openings to optimize screening conditions without simultaneously adversely affecting other screening conditions. While it has been known that different screening conditions prevail at different axial sections of the screen cylinder, e.g. at top and bottom levels of a vertical screen cylinder, it has been suggested that this be corrected by changing the rotor configuration, to increase the distance between the screening medium and the rotor towards the reject end of the screening zone, to inject dilution water into the screening zone, or to feed fiber suspension at different levels into the screening zone in order to optimize screening conditions. However until the invention no economical and successful method has been suggested for providing optimally uniform screening conditions, i.e. optimal screening capacity and cleaning efficiency, along the length of the whole surface of the screen cylinder.
The typical normal goals in screening of wood pulp are the following performance criteria:
high screening efficiency, i.e. efficient screening in which the only rejects are unwanted material (debris), with a minimum of accept fiber content in the reject portion; PA1 high debris removal efficiency, i.e. which leads to an accept portion (flow) which is as close as possible to 100% free from unwanted material (debris) and containing only desirable fibers; and PA1 good runnability at targeted capacity, i.e. the screen operating at a reasonable pressure drop with minimal change in consistency between feed, accept and reject flows. PA1 Smooth surface about 2-4 m.sup.2 /s.sup.2 PA1 PROFILE.RTM. surface about 15-30 m.sup.2 /s.sup.2.
In general all present new screening technology seeks to attain the above goals. One key factor affecting the above goals/performance parameters is the retention time of the fiber suspension to be screened. As the fiber suspension travels through the screen, from the inlet end to the reject end, the time the debris and fibers have spent in the screen increases (the retention time increases). If the screen is run with a low rate of reject out-take, the retention time will further be increased.
Given a predetermined volume V of fiber suspension containing fibers, debris and water, one can say that the longer the retention time inside a screen the more likely will the conventional screen cylinder technology: a) Accept a larger volume of water than fiber. b) See in the accept flow an increasing population (in the volume V) of smaller but still unacceptable debris. [If the 2 or 3-dimensional size of the debris at a certain position along the screen compromises the barrier effect (that is corresponds to the size of the screen openings), the debris will also be accepted by the screen cylinder.] c) Prevent the lower reject end of the screen cylinder from doing useful screening (=good accept flow of fibers) as the fiber density becomes too high for the openings (congestion of fibers will occur in the screening apertures). And, d) increase the pressure drop (dp) as the same predetermined accept flow (capacity) will now have to pass through a smaller "open" portion of the screen cylinder. Higher pressure is required in order to increase velocities in the cylinder openings and an increasing debris population will get "pushed" through the cylinder openings into the accept side.
According to the present invention it has been recognized that the above mentioned differences call for different screening media at different axial portions of the screen in order to achieve optimal screening. It can--by analyzing the elements, i.e. fibers, debris and water in the accept portion of current screening technology--be seen that until now the accept portion has been a compromise of different distributions from various axial sections in the screen cylinder and not an optimal result of screening.
It is desirable to provide an improved average and total debris removal efficiency while at the same time providing improved screening capacity. That is, it is desirable to provide a screen cylinder, a screen, and a method of screening in which simultaneously impurities and other undesirable constituents are more efficiently prevented from flowing into the accept side, and accepts fibers are more efficiently allowed to flow through the screen openings especially in the reject end of the screen.
The present invention provides an improved screen and method of screening fiber suspensions, in which earlier discussed drawbacks are minimized. The present invention provides, especially for the pulp and paper industry, an improved screen and a method of screening cellulose fiber suspensions where screening is performed at improved hydrodynamic screening conditions. The present invention provides a screen cylinder, a screen, and a method of screening fiber suspensions with different and more efficient screening conditions in at the inlet and reject ends of the screen cylinder. And, the present invention provides a screen cylinder, a screen, and a method of screening with improved capacity and runnability of the screen, while at the same time maintaining or improving the debris removal efficiency.
The present invention provides a new non-uniform screening medium for optimized screening results, providing--in the axial direction--multisectional, or gradually, or continuously changing, screen cylinders.
Improvement in screening performance is achieved according to the invention by gradually, or in steps, changing screening characteristics through design/manufacture of the feed side contour, e.g. the feed side surface configuration or the configuration of the inlet side of the screening media openings. This accommodates optimum screening at all portions of the screen cylinder, despite different fiber suspensions and hydrodynamic conditions at its inlet and reject ends.
According to the present invention there is provided a method of screening a fiber suspension using a screening medium disposed in a screen housing having an inlet end and a reject end, the screening medium having screen openings therein with a feed side of the screening medium connected to an inlet for fiber suspension and a rejects outlet, and an outlet side of the screening medium connected to an accepts outlet, and at least a portion of the feed side being contoured. The method comprising the steps of: (a) Introducing a fiber suspension into the inlet end of the screen housing to flow into contact with the feed side of the screening medium, the openings in the screening medium allowing accepts to flow to the outlet side thereof, and causing rejects to flow to the rejects outlet. (b) Discharging rejects from the housing through the rejects outlet. (c) Discharging accepts from the housing through the accepts outlet. And, (d) by providing a different surface configuration of the screening medium feed side adjacent the reject end of the housing compared to adjacent the inlet end of the housing, increasing the turbulence of the fiber suspension at the feed side of the screening medium adjacent the reject end of the screen housing compared to the turbulence adjacent the inlet end of the screen housing to accommodate differences in flow velocity of the fiber suspension through the screening medium openings and the consistency of the fiber suspension at the reject end compared to the inlet end, to make the performance of the screening medium from the inlet end to the reject end more uniform than if the same surface configurations were provided thereat.
The improved method thus utilizes adjacent the reject end of the screening medium a feed side having a more aggressive or higher grade of fluidization inducing surface than adjacent the inlet end. The screening medium may thereby have at its reject end a contoured feed side surface and at its inlet end a smooth feed side surface. Of course adjacent both reject and inlet ends the feed side may be contoured surfaces (with grooves), with differences in turbulence achieved, for example by other differences in configuration of the contoured surfaces. For example variables that may be changed to affect the turbulence include the depth of the grooves, the angle of inclination of the downstream wall of the grooves, the spacing between the grooves, the radius of curvature of the opening edges (see "Suspensions In Process Flows", Kerekes, CPPA Journal of Record, Technical Section, Canada, Apr., 1994, pp. 18, 19), the frequency of openings in grooves, and the dimensions or configuration (i.e. some holes and some slots) of the screening openings in the grooves.
The invention also contemplates a screening cylinder comprising: An axis of elongation. A cylindrical feed side surface having a plurality of first screening openings provided therein. A cylindrical outlet side surface having a plurality of second screening openings therein communicating with the first screening openings. A first end of the surfaces along the axis of elongation, and a second end of the surfaces along the axis of elongation. A first surface configuration of the feed side surface having a first suspension-turbulence-inducing capability adjacent the first end. And, a second surface configuration of the feed side surface having a second suspension-turbulence-inducing capability adjacent the second end, the second capability being at least about 10% greater (and preferably at least about 40% greater, and typically more than 100% greater) than the first capability. As discussed above with respect to the method, the difference in turbulence can be achieved by varying one or more (or all) of the following parameters: the depth of the grooves, the angle of inclination of the downstream wall of the grooves, the spacing between the grooves, the radius of curvature of the opening edges, the frequency of openings in grooves, and the dimensions or configuration (i.e. some holes and some slots) of the screening openings in the grooves.
The measurement of changes in turbulence levels between a feed and reject end of a cylinder is difficult, and there are no present industry standards. The turbulence level of interest is the turbulence in the vicinity of the screen cylinder surface, and not the rotor. Comparison of turbulence levels may be by comparison of the turbulence intensity levels (m.sup.2 /s.sup.2). The situation where there would be the most likely maximum difference in turbulence levels is where the inlet section of the cylinder is smooth, and the reject end is highly contoured, such as for a conventional PROFILE.RTM. screen cylinder. While there are so many variables affecting turbulence, such as outflow/inflow mode, pressure drop, tangential velocity, type of rotor, etc. which are not dependent upon the screen cylinder configurations, a rough estimate of the difference in magnitude of turbulence as a result of the surface configuration of the screening medium is:
That is in this scenario the turbulence adjacent the discharge end would be more than 100% greater than adjacent the inlet end as a result of the surface configuration changes alone (independent of other factors which may ultimately play a part in the total turbulence differential).
The screen cylinder according to the invention is typically provided in combination with a pressure screen housing having an inlet end, and a reject end. The screen cylinder is mounted in the screen housing so that the first end thereof is adjacent the housing inlet end, and so that the second end thereof is adjacent the housing reject end. A rotor is provided for applying pulses to the screen cylinder, the screen cylinder and rotor being rotatably movable with respect to each other. For example the screen cylinder feed side surface is the inner surface of the cylinder, and the rotor is mounted within the cylinder and rotates while the cylinder remains stationary.
Thus according to the invention more aggressive or turbulent screening conditions are created adjacent the reject end of the screening zone. The increased turbulence affects the high concentration fiber suspension in the reject end so that the fibers are separated from each other, thereby increasing the accept fiber flow velocity through the openings in the screening medium. Also, in order to prevent debris and fiber bundles or other non-desirable constituents from flowing through the screen openings at the outlet end of the screening medium the dimensions of the openings, i.e. diameter of round holes, or width of slots, can be decreased if desired. The increased turbulence or fiuidization will allow some decrease in dimensions without the screening capacity being decreased to a negative extent.
In some pulp screens alternating grooves and ridges are provided by e.g. machining on the inlet side of the screening medium for adjusting the flow characteristics of the pulp passing through the screen. The direction of the grooves is preferably transverse to the direction of the flow of fiber suspension. In PROFILE.RTM. contoured screen cylinders, such as shown in U.S. Pat. No. 4,529,520 (the disclosure of which is hereby incorporated by reference herein), the upstream side plane of the grooves is at one angle (e.g. substantially perpendicular, e.g. about 85.degree.-120.degree.) to the envelope surface of the screening medium, whereas the downstream side plane of the grooves is inclined at another angle (e.g. forming a 60.degree.-5.degree. angle with the envelope surface). The screening openings are formed in (adjacent or completely within) the bottom portions of the grooves. The PROFILE.RTM. contoured screen cylinders with specifically shaped grooves induces high-intensity turbulence at the screening openings (holes or slots), and decreases the resistance to fluid flow through the screening medium, i.e. provides a smooth flow of accept fiber suspension through the screening medium. Thus a PROFILE.RTM. screen cylinder can run at higher consistencies, lower pressure drop, and with less fractionation of long fibers.
In constructing a PROFILE.RTM. screen cylinder, a small tilted angle from ninety degrees for the upstream side plane of the groove (e.g. up to about a +30 degree tilt) helps to induce a higher flow intensity toward the aperture, however at the cost of lower debris removal efficiency. The balance between capacity and accepts cleanliness can then be handled by changing slightly the angle or the aperture size. In any event, when viewing any particular groove of such cylinders from a perpendicular position above the surface, the upstream side plane appears to clearly closer to a perpendicular surface (even when tilted, eg. up to about thirty degrees) than the downstream inclined plane.
According to a preferred embodiment of the invention, when utilizing the above described PROFILE.RTM. screen cylinder, the grooves at the reject end of the screening medium may be about 0.2 to 2 mm, preferably 0.3 to 1.2 mm, deeper than the grooves adjacent the inlet end thereof in order to induce an increased (by at least about 10%, preferably at least about 40%, typically more than 100%) turbulence in the vicinity of the screen surface adjacent the reject end.
According to another preferred embodiment of the invention a difference in turbulence induced by the grooved contour configuration can be achieved by varying the steepness of the side walls of the grooves. The steeper the downstream side wall of the grooves or the smaller the angle between the upstream and downstream side walls the more turbulence will be induced on the fiber suspension at the screening openings. Of course all factors of the contour configuration cooperate, so that the effect of some changes in configuration may override others. If e.g. the grooves at the reject end are much deeper than the grooves at the inlet end, then it may be possible, if for some reason desired, to use grooves with less inclined downstream side walls at the reject end than at the inlet end and still achieve sufficient turbulence at the reject end.
The present invention provides, according to still another preferred embodiment thereof, the possibility to use different hole or slot sizes, or different types of holes or slots, adjacent the inlet and reject ends of the screening medium. The probability of debris being accepted by the screen increases towards the reject end of the screening medium, as the concentration of debris increases. Also a longer retention time increases the probability of debris being accepted. It may be desirable to use smaller holes or slots, adjacent the reject end in order to increase the barrier effect of the screen and compensate for the increased tendency of debris to flow through openings. Increased turbulence at the reject end, for increasing screening capacity, also prevents clogging of the smaller openings. Slot size (or round hole diameter) may vary so that slots in the reject end are about 10-100+% (preferably about 20-50%) narrower than those in the inlet end. For example slot size may in a slotted screen plate thereby vary e.g. between about 0.1-0.5 mm, e.g. between about 0.1-0.2 mm, at the reject end, and about 0.35-0.5 mm at the inlet end.
It may also be possible to achieve different screening conditions by having different types of openings in the screening medium at the inlet end of the screen than at the reject end of the screen, i.e. round holes at the feed end and slots at the reject end.
It may be desirable to adjust spacing between grooves and/or openings in contoured surfaces or spacing between openings in smooth surfaces or the width of the bottom of grooves in accordance with changed screening conditions. For example by providing a spacing adjacent the reject end at least about 10% greater than at the inlet end a significant difference in turbulence can be achieved. That is adjacent the inlet end about 7-9 grooves per inch (e.g. 8) are provided, while adjacent the reject end about 5-7 (e.g. 6) grooves per inch, and at least about one groove per inch less, are provided.
It is appreciated that the invention may be employed with screening cylinders in which the fiber suspension flows from inside the cylinder to the outside as well as in screening cylinders in which the fiber suspension flows from the outside into the cylinder.
The invention may be employed with screen cylinders made by bending or forming screen plates having grooves provided by machining into the plates, or by welding bars or similar surface elevations onto the surfaces thereof. The present invention may also be employed with screening cylinders made of parallel rods with slots therebetween, or screening cylinders with cylindrical wedge wire sections. Turbulence of the reject end of the screening medium may be increased by welding contour increasing elements on the surface of the screening medium.
The present invention provides a screening technology according to which it is possible to change the screening characteristics gradually by changing, e.g. the screen contour parameters gradually, not only in two steps but if desired in several steps (e.g. 3, 4, or more steps) or essentially continuously (e.g. from groove to groove), for maintaining optimum hydrodynamic/screening conditions at any axial section of the screening device. Also the screening openings in grooved screen plates may be provided in different patterns; for example one row of openings in some grooves, zero or two rows of openings in the adjacent grooves, etc.
The invention provides optimized screening concepts preventing high concentration fiber layers from being formed on the screening medium at the reject end, thereby leading to a more uniform accept flow through the screening medium and a more uniform and higher capacity distribution over the screen cylinder. The invention especially makes it possible to operate screens at higher consistencies and debris concentrate levels, without sacrificing debris removal efficiency.
The present invention provides a substantial improvement in screening with both contoured (e.g. grooved) screen cylinders, and conventional smooth screen cylinders, and provides potential improvements in capacity and cleanliness, also at lower reject rates.
In summary, it may be the that more efficient debris removal may be maintained by utilizing smaller screen openings adjacent the reject end of the screening medium, while capacity at the same time is maintained or even increased by inducing higher turbulence or fluidization adjacent that end. The decreased size of openings is compensated for by the increased turbulence or fluidization.
The present invention seeks to provide in one single screen or screen cylinder low concentration screening conditions adjacent the inlet end and high concentration screening conditions adjacent the reject end. It seeks to make it possible in many processes to achieve the same screening result, and same screening capacity at maintained cleaning efficiency, with one single screen or screen cylinder as with two separate primary and secondary conventional screens or screen cylinders, the first with low debris concentration and the other with high debris concentration, having conventional uniform screen cylinders.
The invention will be described in more detail in the following with the reference to the accompanying drawings.